Adaptive stability control for a driver circuit

ABSTRACT

A circuit for driving a load may include a control loop having a response characteristic. A headroom signal indicative of the headroom voltage of the circuit may set one or more parameters of the response characteristic. A load sign indicative of electrical loading on the circuit may further set the response characteristic.

BACKGROUND

Unless otherwise indicated, the foregoing is not admitted to be prior art to the claims recited herein and should not be construed as such.

With the integration of high resolution cameras into mobile devices such as smart phones and computing tablets, high current (e.g., >1 A) flash LEDs are typically required for high pixel (e.g., >5M pixel) cameras. Thus, the power dissipation in power management ICs (PMICs) that include flash LED drivers is elevated. Since the flash LED driver is one of the higher power consumption modules in a PMIC, minimizing the power dissipation from the flash LED driver is an important consideration in a PMIC design to extend battery life and reduce thermal risks.

Typically, a flash LED driver includes an output stage and a current regulator that regulates the output current of the output stage. The current regulator may include an error amplifier that is connected to the output stage in a feedback loop.

Power dissipation can be reduced by reducing the headroom voltage of the flash LED driver. Headroom voltage refers to the voltage drop between the driver's voltage supply and the output voltage of the driver. However, when the headroom voltage is decreased, the feedback loop tends to drive the output stage into the linear region, thus reducing the system loop gain. Conversely, when the headroom voltage is increased, the output stage is driven into the saturation region with much larger gain (e.g., 50-60 dB or higher), which reduces phase margin and thus system stability.

Further exacerbating the problem is that conventional PMIC designs cannot anticipate what devices (smart phones, computer tablets, portable cameras, etc.) the PMICs will be used in, and how such devices will be used by the end user. Accordingly, a given PMIC can be exposed to a wide range of headroom voltage conditions and load conditions and so its design is not likely to be adequate for all use cases.

SUMMARY

In various embodiments, a circuit may include a driver stage for driving an external load. The driver stage may be connected in a control loop configuration with a control circuit. In some embodiments, a headroom sensor may provide a headroom signal to control a response characteristic of the control loop. In other embodiments, a load sensor may provide a load signal to further control the response characteristic of the control loop.

In some embodiments, the gain of the response characteristic may be varied according to the headroom signal. In other embodiments, a bandwidth of the response characteristic may be varied according to the headroom signal. In a particular embodiment, the headroom signal may vary the dominant pole location of the response characteristic.

In some embodiments, an internal zero of the response characteristic may be set by the load signal.

The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

With respect to the discussion to follow and in particular to the drawings, it is stressed that the particulars shown represent examples for purposes of illustrative discussion, and are presented in the cause of providing a description of principles and conceptual aspects of the present disclosure. In this regard, no attempt is made to show implementation details beyond what is needed for a fundamental understanding of the present disclosure. The discussion to follow, in conjunction with the drawings, make apparent to those of skill in the art how embodiments in accordance with the present disclosure may be practiced. In the accompanying drawings:

FIG. 1 shows an example of a high level block diagram of a circuit in accordance with the present disclosure.

FIGS. 2, 2A, and 2B illustrate frequency response curves.

FIG. 3 shows an illustrative embodiment in accordance with the present disclosure.

FIG. 3A shows some details for a load sensor.

FIG. 4 shows some details for an illustrative embodiment of OP amp 304.

FIG. 4A shows an alternative embodiment of FIG. 4.

FIGS. 5A-5D illustrate frequency response curves that characterize aspects of the present disclosure.

FIGS. 6A-6D illustrate frequency response curves that characterize further aspects of the present disclosure.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be evident, however, to one skilled in the art that the present disclosure as expressed in the claims may include some or all of the features in these examples, alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein.

FIG. 1 shows a circuit 100 in accordance with the present disclosure. In some embodiments, the circuit 100 may comprise a driver stage 112 having a drive output 112 a that can be connected to drive a load 10. In general, the load 10 may be any kind of a load. In some embodiments, the load 10 may be an LED flash component of a camera; e.g., a digital camera or a camera in a portable computing device such as a computer tablet, a smartphone, and so on.

A control circuit 114 may provide a control signal 114 a that can be used to control operation of the driver stage 112. In some embodiments, the control circuit 114 may be configured with the driver stage 112 as a feedback control loop 110, with the driver output 112 a feeding back to an input (e.g., 1^(st) input) of the control circuit.

In accordance with present disclosure, a response characteristic of the control loop 110 may be set or otherwise altered in accordance with a signal indicative of the headroom voltage of the circuit 100 during operation when the circuit is driving the load 10. Headroom voltage (“headroom”) refers to the voltage drop between the driver's voltage supply (e.g., V_(IN)) and the output voltage (e.g., V_(OUT)) of the driver 112; e.g., V_(IN)-V_(OUT). Accordingly, circuit 100 may include a headroom sensing circuit 116 that senses a headroom of the circuit. The headroom sensing circuit 116 may produce a headroom signal that is indicative of or otherwise representative of the headroom voltage of circuit 100. The headroom signal can be provided to another input (e.g., 2^(nd) input) of the control circuit 114 to affect the response characteristic of the control loop 110. This aspect of the present disclosure will be discussed in more detail below.

In some embodiments, the response characteristic of the control loop 110 may be further set or otherwise altered in accordance with a signal indicative of electrical loading on the circuit 100 due to load 10 during operation; e.g., electrical loading may be represented by the current flowing through the load. Accordingly, circuit 100 may include a load sensing circuit 118 that can generate a load signal indicative of or otherwise representative of the electrical loading on the circuit. The load signal can be provided to another input (e.g., 3^(rd) input) of the control circuit 114 to affect the response characteristic of the control loop 110. This aspect of the present disclosure will be discussed in more detail below.

Referring to FIG. 2, the response characteristic of the control loop 110 (FIG. 1) may be represented by a frequency response curve 200 (e.g., a Bode plot). The response characteristic of the control loop 110 may be characterized by a gain (sometimes referred to as the closed loop gain) that varies with frequency, as illustrated in the frequency response curve 200. The response characteristic of the control loop 110 may be further characterized by a bandwidth (f_(bandwidth)), which is conventionally defined as the frequency at which the closed loop gain falls by −3 dB. As will be appreciated by those of ordinary skill, the plot shown in FIG. 2 is a straight-line plot of the actual response characteristic, which is continuous and has a 3 dB drop-off at the pole position P_(D).

The frequency response curve 200 shown in FIG. 2 represents poles P_(D), P_(L) of the response characteristic of the control loop 110. It will be appreciated, that in general, the response characteristic of control loop 110 may include any number and combination of poles and zeroes. For example, the response characteristic of control loop 110 may be expressed as a transfer function represented by the following Laplace transform:

${{H(s)} = {G{\prod\frac{\left( {s - x_{n}} \right)^{a_{n}}\;}{\left( {s - y_{n}} \right)^{b_{n}}}}}},$

where

-   -   H(s) is the transfer function,     -   G is the closed loop gain,     -   x_(n) and y_(n) are constants,     -   s is the complex frequency jω, and     -   a_(n) and b_(n) are >0.         A pole exists for every value of s where ω=y_(n) and similarly a         zero exists for every s where ω=x_(n). The disclosed embodiments         describe the behavior of the response characteristic at poles         P_(D) and P_(L), and at a zero relating to pole P_(L). However,         it will be appreciated from the teachings set forth in the         present disclosure that embodiments in accordance with the         present disclosure need not be restricted to the poles and         zeroes specifically disclosed in the present disclosure, and         that other embodiments may involve other poles and/or zeroes of         the response characteristic of the control loop 110.

The lowest frequency pole P_(D) is sometimes referred to as the “dominant pole” because it dominates the effect of any higher frequency poles. As noted above, the dominant pole typically defines the bandwidth of the response characteristic of the control loop 110.

The load 10 that is driven by the driver stage 112 (FIG. 1) can affect the response characteristic of the control loop 110. The load 10 introduces an external pole P_(L) to the frequency response curve of the control loop 110. The location or position of external pole P_(L) depends on the size of the load 10 (e.g., expressed as an impedance Z). This effect is illustrated in FIGS. 2, 2A, and 2B for different sized loads.

FIGS. 2, 2A, and 2B further illustrate that as the size of the load 10 increases (e.g., increasing impedance Z), the external pole P_(L) moves closer to the dominant pole P_(D). Generally, each non-dominant pole closer than a decade away from the unity gain bandwidth of the feedback loop will reduce phase margin, which can ultimately degrade the control loop stability. If the external pole P_(L) is sufficiently separated from the dominant pole P_(D), the effect of the external pole on stable operation of the control loop 110 can be small; e.g., the location of external pole P_(L) in FIG. 2 may be sufficiently far away from dominant pole P_(D) so that the control loop 110 will be stable. However, in FIG. 2B, the external pole P_(Lb) may be close enough to the dominant pole P_(D) that the phase margin is so small that unstable operation can result.

Referring to FIG. 3, in a particular embodiment, the load may be an LED flash unit 30, such as might be found in digital cameras, portable computing devices, smart phones, etc. The driver stage 112 may comprise a transistor F₂, such as a power MOSFET for example. The transistor P₂ may provide a drive current from the supply voltage V_(IN) to the LED flash unit 30.

The control circuit 114 may comprise an OP amp 304 and a constant current source I_(REF). The control circuit 114 may further include a current mirror, comprising a transistor P₁ and the driver stage transistor P₂, that is driven by the output V_(DRVR) of OP amp 304.

The driver stage 112 and control circuit 114 may constitute the control loop 110. The OP amp 304 in control circuit 114 may control the driver stage transistor P₂ in a feedback loop by regulating the drive current (provided as feedback voltage V_(FB)) based on a reference provided by the constant current source I_(REF) (provided as V_(REF)). In some embodiments, the constant current source I_(REF) may be a small current source (e.g., 10 μA). Accordingly, the device dimensions of P₁ and P₂ in the current mirror may be designed to provide a suitable P₁/P₂ current ratio in order to provide a suitable drive current to operate the LED flash unit 30. In an embodiment, for example, the P₁/P₂ current ratio may be 1:10,000, giving a current gain of 10,000 at P₂. It will be appreciated of course that in other embodiments, P₁ and P₂ can be designed to provide any suitable current gain.

As mentioned above, the headroom sensing circuit 116 may generate a headroom signal indicative of or otherwise representative of the headroom voltage of the circuit 100. In some embodiments, the headroom sensing circuit 116 may comprise a comparator 302 to compare the driver stage output voltage V_(OUT) provided to the load (e.g., LED flash unit 30) against the supply voltage (e.g., V_(IN)). The headroom signal HDRM_out produced by the headroom sensing circuit 116 is thus representative of the headroom voltage of the circuit 100 and may feed into the control circuit 114. In some embodiments, HDRM_out feeds into a control input IN₁ of the OP amp 304 of control circuit 114.

In some embodiments, resistor divider networks may be used to reduce the voltage levels that feed into comparator 302. For example, the output voltage V_(OUT) on the load 30 may be divided down by the resistor divider comprising R₄ and R₅. The supply voltage V_(IN) can be divided down in similar fashion as described below.

The headroom sensing circuit 116 may include a programmable current source I_(BIAS). The headroom sensing circuit 116 may include a resistor R₁ to drop the supply voltage V_(IN) by an amount, V_hdrm, that is determined by the current source I_(BIAS) and resistor R₁ (e.g., V_hdrm=I_(BIAS)×R₁). Headroom detection may then be determined based on the reduced supply voltage level, namely (V_(IN)−V_hdrm). The voltage level V_hdrm can be viewed as defining a headroom voltage threshold. Resistors R₂ and R₃ then divide down the headroom voltage threshold. The inset in FIG. 3 defines the constant K, which in a particular embodiment also sets forth a relationship among the resistance values of resistors R₁-R₅.

As noted above, the load that is being driven by the driver stage 112 can change, depending what the load is, how it is being used, etc.; for example, an LED flash device 30 can operate in different modes; e.g., flash, strobe, etc. As will be explained below, the size of the load can affect the response characteristic of the control loop 110. In some embodiments, the load sensing circuit 118 may produce a signal Ld_sns_out that represents the drive current from the driver stage 112 that can feed into the control circuit 114. In some embodiments, the Ld_sns_out signal feeds into a control input IN₂ of the OP amp 304 of control circuit 114.

As an example, in some embodiments, the load sensing circuit 118 may comprise a current sensor. Referring to FIG. 3A, for example, load sensing circuit 118′ may comprise a mirrored sense transistor P_(sense) that mirrors the current flow through the driver stage transistor P₂. A comparator 306 may compare the current flow through P_(sense) against a reference and output a signal Ld_sns_out that represents the level of the drive current flowing into the load 30. In other embodiments, the load sensing circuit 118 may be load current programming circuit (not shown) that programs the load current, and the Ld_sns_out signal may be based on a parameter (e.g., stored in a programming register) used to program the load current.

In accordance with the present disclosure, the response characteristic of the control loop 110 can be set based on the headroom signal HDRM_out, or the load signal Ld_sns_out, or both. For example, the HDRM_out signal may be used to set the gain of the response characteristic, or the bandwidth of the response characteristic, or both. Similarly, the Ld_sns_out signal may be used to set the position of a zero of the response characteristic. As will be discussed below, this can improve stability in the control loop 110 as changes in the headroom voltage and loading conditions (e.g., current load) occur during operation of circuit 100.

Referring now to FIG. 4, additional detail of OP amp 304 illustrates how the HDRM_out and Ld_sns_out signals may be used to set the response characteristic of control loop 110. In some embodiments, for example, the response characteristic of the control loop 110 may be set by setting circuit elements in the OP amp 304.

The OP amp 304 may comprise an opamp device 404 having an inverting input and a non-inverting input. The reference voltage V_(REF) may be connected to the non-inverting input through resistor R_(B). The feedback voltage V_(FB) may be connected to the inverting input through resistor R_(A). The opamp device 404 may include a gain control input G that varies the gain of the opamp device. The HDRM_out signal may be coupled directly to or otherwise connected to the gain control input G, thereby allowing the HDRM_out signal to control the gain of the opamp device 404. Accordingly, circuit element Z₁ may represent a wire for a direct connection of the HDRM_out signal to the gain control input G, or Z₁ may represent some intervening electronic circuitry that can provide gain control as some function of the HDRM_out signal. Similarly, circuit element Z₂ may represent a wire or some appropriate intervening electronic circuitry.

The OP amp 304 may include a compensation network 402 of any suitable design. In some embodiments, the response characteristic (e.g., transfer function H(s)) of the control loop 110 may be expressed in terms of the circuit elements comprising the compensation network. In accordance with the present disclosure, the compensation network 402 may comprise one or more circuit elements that can be adjusted or otherwise varied using the HDRM_out signal and/or the Ld_sns_out signal in order to vary the response characteristic of control loop 110.

Merely as an illustration of principles of the present disclosure, consider the compensation network shown in FIG. 4. The compensation network 402 may comprise a resistor element R_(C) connected in series with a capacitor element C_(C1), and a shunt capacitor C_(C2) connected in parallel with the R_(C)/C_(C1) pair. The resistor element R_(C) may provide a selectable resistance. Setting the resistance value of resistor R_(C) can set the position of a zero in the response characteristic of control loop 110. Similarly, the capacitor C_(C2) may provide a selectable capacitance. Setting the capacitance value of capacitor C_(C2) can set the position of the dominant pole in the response characteristic of control loop 110.

In accordance with the present disclosure, the HDRM_out signal may be coupled directly to or otherwise connected to a selector input of the variable capacitor C_(C2). As explained above, circuit element Z₁ may represent a wire or some appropriate intervening electronic circuitry and likewise, circuit element Z₃ may represent a wire or some intervening electronic circuitry. Accordingly, the HDRM_out signal may be used to set the dominant pole position of the response characteristic of control loop 110.

Further in accordance with the present disclosure, Ld_sns_out signal may be coupled directly or otherwise connected to a selector input of the variable resistor R_(C). Circuit element Z₄ may represent a wire for a direct connection of the Ld_sns_out signal to the selector input of the variable resistor R_(C), or Z₄ may represent some intervening electronic circuitry that can set resistor R_(C) as some function of the Ld_sns_out signal. Accordingly, the Ld_sns_out signal may be used to set a zero position of the response characteristic of control loop 110.

In some embodiments circuit elements Z₁-Z₄ are wires representing straight through connections, such as illustrated for example in FIG. 4A.

Referring to FIGS. 5A-5D, frequency response curves 502-508 can be used to illustrate the effects on the response characteristic of control loop 110 when the headroom signal HDRM_out is applied in accordance with the present disclosure. Suppose the frequency response curve 502 in FIG. 5A represents a response characteristic of control loop 110 having a gain G1 and bandwidth f_(b) as determined by the dominant pole P_(D). FIG. 5A illustrates that the headroom signal HDRM_out can vary the gain and/or the dominant pole position (and hence the bandwidth) of the response characteristic.

FIG. 5B, shows how the frequency response curve 504 changes if just the gain is varied, for example, if the gain is reduced to G₂<G₁. Thus, in accordance with some embodiments of the present disclosure, when the headroom voltage exceeds a threshold voltage (e.g., V_hdrm in FIG. 3), the HDRM_out signal can be used to reduce the gain in order to improve stability in the control loop 110. Referring for a moment to FIG. 4, the HDRM_out signal can control the gain of the opamp device 404 to vary the gain of the frequency response curve. Reducing the gain brings the unity gain bandwidth of the control loop 110 back from frequency f₁ to frequency f₂. This improves stability if non-dominant poles exist within a decade or closer to f₁.

FIG. 5C shows how the frequency response curve 506 changes if just the dominant pole is varied, for example, from f_(b) to f_(b)′<f_(b). Since the dominant pole determines the bandwidth, varying the dominant pole (e.g., P_(D) to P_(D)′) has the effect of varying the bandwidth of the response characteristic. Thus, in accordance with some embodiments of the present disclosure, when the headroom voltage exceeds the threshold voltage, the HDRM_out signal can be used to reduce the bandwidth in order to improve stability in the control loop 110. Conversely, when the headroom voltage falls below the threshold voltage, the HDRM_out signal can be used to increase the bandwidth to improve the response time of the control loop 110. Referring again to FIG. 4, the HDRM_out signal can control the value of the variable capacitor C_(C2) to vary the dominant pole position of the frequency response curve.

Referring to FIG. 5D, in some embodiments, the HDRM_out signal may control both the gain and the dominant pole position (i.e., bandwidth) of the response characteristic of control loop 110. The frequency response curve 508 shown in FIG. 5D illustrates an example in which the gain and bandwidth are reduced. Thus, in accordance with some embodiments of the present disclosure, when the headroom voltage exceeds the threshold voltage, the HDRM_out signal can be used to reduce both the gain and the bandwidth of the response characteristic in order to improve stability in the control loop 110. Conversely, when the headroom voltage falls below the threshold voltage, the HDRM_out signal can be used to increase the gain and bandwidth to improve the response time of the control loop 110.

In some embodiments, the gain and bandwidth may change together, as HDRM_out changes. In other embodiments, the gain and bandwidth may be varied independently of each other. For example, circuit elements Z₁-Z₃ (FIG. 4) may be suitable control circuitry to isolate the HDRM_out signal from the gain control input and capacitor C_(C2) for independent operation.

Referring to FIGS. 6A and 6B, frequency response curves 602, 604 can be used to illustrate the effects on the response characteristic of control loop 110 when the load signal Ld_sns_out is applied in accordance with the present disclosure. As explained above, the load 10 (e.g. LED flash unit 30, FIG. 3) introduces an external pole that can affect the response characteristic of the control loop 110. Consider the illustration in FIG. 6A, for example. Suppose the response characteristic of the control loop 110 without a load is represented by the frequency response curve 602. When a load is added, an external pole P_(L) due to the load may be introduced, which may result in frequency response curve 604. The additional drop off occurring a frequency f_(L) reduces stability of the control loop 110 due to a decrease in phase margin.

However, in accordance with the present disclosure, the Ld_sns_out signal may be used to set a zero position of an internal zero in the control loop 110 to compensate for the external pole P_(L). Referring for a moment to FIG. 4, the Ld_sns_out signal can control the value of variable resistor R_(C) to vary an internal zero of the response characteristic. This effect is illustrated in FIG. 6B, where the Ld_sns_out signal can set the zero position of an internal zero Z_(L) to cancel the effect of the external pole P_(L) and eliminate the drop off at frequency f_(L) to improve stability.

As the load changes, so will the pole position of the external pole, as illustrated in FIG. 6C. For example, in one situation the load may be such as to introduce external pole P_(a) to the response characteristic; in another situation, the load may be such as to introduce a different external pole, e.g., P_(b), to the response characteristic, and so on. Each time the external pole changes, the response characteristic of the control loop 110 changes. The stability of the control loop 110 can therefore vary according to the load, as illustrated by the frequency response curves, 612 a, 612, b, 612 c, 612 d. As one can appreciate, designing a control circuit 114 (FIG. 1) to maintain stability under any load condition can be challenging, since the designer has no control over what load the circuit 100 will be connected to or how the load may change during operation.

In accordance with the present disclosure, the load sensor 118 can produce a Ld_sns_out signal that is representative of or otherwise tracks the load. The Ld_sns_out signal can be used to adjust an internal zero of the control loop 110 (e.g., via variable resistor R_(C)) to compensate for varying external poles resulting from varying loads, and thus improve stability in the control loop 110 as represented by the frequency response curve 612. This effect is illustrated in FIG. 6D, which illustrates that the internal zero can vary to track the external pole as the load varies.

Advantages and Technical Effect

Aspects of circuits in accordance with the present disclosure allow for automatic adjustment of the loop gain and/or dominant pole position of the control loop based on voltage headroom during operation, when a load is being driven.

Another aspect of circuits in accordance with the present disclosure allow for automatically varying the zero position of an internal zero of the control loop based on load condition of the load being driven by the circuit.

The above description illustrates various embodiments of the present disclosure along with examples of how aspects of the particular embodiments may be implemented. The above examples should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the particular embodiments as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents may be employed without departing from the scope of the present disclosure as defined by the claims. 

We claim the following:
 1. A circuit comprising: a driver stage having an input and further having an output for a drive signal that can drive an external load; a headroom sensing circuit having an output for a headroom signal generated by the headroom sensing circuit, the headroom signal representative of a headroom voltage of the circuit; and a control circuit having a first input and an output for a control signal generated by the control circuit, the control circuit and the driver stage connected together to define a control loop, wherein the output of the driver stage is connected to the first input of the control circuit and the output of the control circuit is connected to the input of the driver stage, the control circuit having a second input connected to the output of the headroom sensing circuit, wherein a response characteristic of the control loop is set based on the headroom signal received from the headroom sensing circuit.
 2. The circuit of claim 1 wherein the received headroom signal sets a gain of the response characteristic of the control loop.
 3. The circuit of claim 1 wherein the received headroom signal sets a bandwidth of the response characteristic.
 4. The circuit of claim 1 wherein the received headroom signal sets a pole position of a frequency response curve representative of the response characteristic of the control loop.
 5. The circuit of claim 4 wherein the received headroom signal sets a dominant pole position of a frequency response curve representative of the response characteristic of the control loop.
 6. The circuit of claim 1 wherein the control circuit comprises an amplifier stage and a compensation stage, wherein circuit elements comprising the compensation stage determine the response characteristic of the control loop, wherein at least one of the circuit elements of the compensation stage is selectable using the headroom signal to vary a bandwidth of the control circuit.
 7. The circuit of claim 6 wherein the amplifier stage includes a gain control input, wherein the headroom signal is provided to the gain control input to set a gain of the amplifier stage.
 8. The circuit of claim 1 further comprising a load sensing circuit having an output for a load signal representative of an electrical loading on the circuit, the control circuit further having a third input connected to the output of the load sensing circuit, wherein the response characteristic of the control loop is further set based on the load signal received from the load sensing circuit.
 9. The circuit of claim 8 wherein the received load signal sets a zero position of a zero of a frequency response curve representative of the response characteristic of the control loop.
 10. The circuit of claim 8 wherein the electrical loading is a current flow from the output of the driver stage to a load connected to the driver stage.
 11. The circuit of claim 1 wherein the headroom sensing circuit is connected to a voltage supply of the driver stage and the output of the driver stage.
 12. The circuit of claim 11 wherein the headroom sensing circuit includes a threshold setting circuit.
 13. A circuit comprising: first means for generating an electric current to drive a load; second means for controlling the first means, the first means and the second means configured as a control loop; and third means for sensing a voltage headroom of the circuit and generating a headroom signal representative of the voltage headroom, wherein the third means is connected to the second means to adjust a response characteristic of the control loop using the headroom signal.
 14. The circuit of claim 13 wherein the received headroom signal sets a gain of the response characteristic of the control loop.
 15. The circuit of claim 13 wherein the received headroom signal sets a bandwidth of the response characteristic of the control loop.
 16. The circuit of claim 13 further comprising fourth means for sensing a flow of the electric current from the first means and generating a load signal representative of the flow of the electric current from the first means, wherein the fourth means is connected to the second means to further adjust the response characteristic of the control loop using the load signal.
 17. The circuit of claim 16 wherein the load signal sets a zero position of a frequency response curve representative of the response characteristic of the control loop.
 18. A circuit comprising: a driver stage having an input and further having an output for a drive signal that can drive an external load; a load sensing circuit having an output for a load signal generated by the load sensing circuit, the load signal representative of electrical loading by the external load on the circuit; and a control circuit having an input and an output for a control signal generated by the control circuit, the control circuit and the driver stage connected together to define a control loop, wherein the output of the driver stage is connected to the input of the control circuit and the output of the control circuit is connected to the input of the driver stage, wherein the output of the load sensing circuit is connected to a control input of the control circuit, wherein a response characteristic of the control loop is set based on the load signal received from the load sensing circuit.
 19. The circuit of claim 18 wherein the electrical loading is an electric current load.
 20. The circuit of claim 18 wherein the load signal sets a zero position of a zero of a frequency response curve representative of the response characteristic of the control loop.
 21. The circuit of claim 18 further comprising a headroom sensing circuit having an output for a headroom signal representative of a headroom voltage of the circuit, the output of the headroom sensing circuit connected to another control input of the control circuit, wherein the response characteristic of the control loop is further set based on the headroom signal received from the headroom sensing circuit.
 22. The circuit of claim 18 wherein the received headroom signal sets either or both a gain of the response characteristic of the control loop or a bandwidth of the response characteristic. 